Knowing the optical characteristics of a cornea can be very helpful to an ophthalmologist. Specifically, by knowing the refractive power of the cornea the ophthalmologist is able to diagnose certain corneal diseases, such as Keratokonus, and to properly plan for refractive surgical operations for the correction of such maladies as astigmatism and myopia. Heretofore, the determination of the refractive power of a cornea has been limited, and based solely on an evaluation of the front or anterior surface of the cornea. Such limited evaluations have, unfortunately, obtained a rather high level of unsatisfactory surgical results. It is clear that a more complete determination of the refractive topography of the cornea is necessary if a greater percentage of surgeries are to be successful. As can be easily appreciated, an evaluation of the entire cornea, rather than only its anterior surface, is both necessary and desirable.
Anatomically, the cornea of an eye comprises five discernible and distinct layers of tissue. Going inwardly from the anterior surface of the cornea to its posterior surface these layers are, in order: the epithelium; Bowman's membrane; the stroma; the endothelium; and Descemet's membrane. Of these, the stroma is, by far, the largest volume of tissue in the cornea. Consequently, the stroma is the most significant corneal tissue insofar as a contribution to the refractive power of the cornea is concerned. It follows, therefore, that surgical operations to alter the refractive power of the cornea should be undertaken with as much knowledge of the stromal contribution as is possible. Importantly, it is now known there are stress distributions within the stroma which, if unaccounted for, can lead to unpredictable results when they are relieved by an incision. Stated differently, the relaxation of stress distributions in the stroma by a corneal incision affects the surgical reshaping of the cornea.
It is known that stress distributions within the stroma result in birefringence. Further, it is known that the birefringent properties of the corneal stroma which result from these stresses are detectable. Consequently, by obtaining a topography of the intrinsic birefringence of the cornea, the stress distribution within the cornea can be ascertained. With this information, incisions can then be made into the cornea which will give more predictable results for the refractive power of the surgically corrected cornea.
To determine the birefringent topography of the cornea, measurements can be made by an ellipsometer of a type fully disclosed in U.S. application Ser. No. 08/709,243 which is incorporated herein by reference, and which is assigned to the same assignee as the present invention. These measurements, however, are affected by several factors. One such factor depends on the angle of incidence (.theta.) at which the ellipsometer's light beam enters the cornea relative to its anterior surface. In turn, the angle of incidence .theta. is affected by the curvature (topography) of the anterior corneal surface. Additionally, other factors which cause corneal movement, such as the patient's breathing, and the patient's heart beat, will have their effects and need to be considered. Thus, in order to obtain an accurate and precise birefringent topography which will be useful to the ophthalmologist for reshaping the cornea, the ellipsometer's birefringent measurements need to be corrected to properly account for these factors.
In light of the above it is an object of the present invention to provide a system and method for determining the birefringent topography of a cornea which accounts for local cornea curvature influences in the determination of the topography. It is another object of the present invention to provide a system and method for determining the birefringent topography of a cornea which accounts for the patient's heart beat and the patient's breathing in the determination of the topography. Yet another object of the present invention is to provide a system and method for determining the birefringent topography of a cornea which is easy to use, relatively simple to manufacture and comparatively cost effective.